Which of these is true about pure substances?

a. they can only contain one type of atom
b. they can only contain one type of molecule
c. they may contain one type of atom or one type of molecule
d. they can contain different types of atoms and molecules

Why is rain likely when warm, moisture-laden air meets cold air? a) the lighter warm air will rise and cool down, causing condensation and rain. b) the cold air moves faster and pushes the warm air away, causing condensation and rain. c) the moisture in the warm air condenses on contact with the cold air, causing rain to fall. d) the cold air mixes with the warm air, reducing its temperature causing moisture to condense.